The application of protective and decorative coatings in the nature of paint or varnish by electrophoretic deposition now is a large industry. Quite commonly such coating processes are called "electrocoating". U.S. Patent and Trademark Office Class 204, subclass 181, reflects the large growth of this technology in recent years.
Generally, in an electrocoating process of the type concerned here, one or more cathode workpieces and one or more counterelectrodes (anodes) are maintained for a short period of time in an electrical circuit with a dilute aqueous dispersion of film-forming paint binder, usually pigmented, between them. In most such coating operations a net unidirectional electric current is passed between these electrodes at fair voltage (e.g., 50+ V.). Generally such current is rectified ac current. This causes deposition of the coating on the electrodes (workpieces). Most frequently the binder dispersion is maintained as a bath in which the electrodes are at least partially immersed. Other methods for forming the electrical circuit also have been proposed, e.g., by showering the workpieces with continuous streams of the current-carrying aqueous coating dispersion.
Representative of the earliest practical electrocoating is that shown in Allan E. Gilchrist's U.S. Pat. No. 3,230,162 of 1966. More recently cationic or cathodic electrocoating has become popular. U.S. Pat. No. 3,799,854 and many subsequent patents involving the electrodeposition of blocked isocyanate and amino resins onto a cathodic workpiece, and the subsequent curing of the electrodeposited film into a crosslinked structure are representative of such cathodic electrocoating. These very practical techniques employ comparatively low molecular weight, structurally ionized resinous material as their fundamental paint binders for the electrocoating, in contrast to the instant latices made by emulsion polymerization.
That is not to say that the cathodic electrodeposition of synthetic latices has not been proposed before. Representative cathodic electrocoating proposals using synthetic latex binders include those of these U.S. Pat. Nos. 3,873,488; 3,882,009; 3,994,792; 3,998,776; 4,017,372; 4,225,406; and 4,255,407.
Synthetic latices made by emulsion polymerization as cathodic electrocoating binders have been regarded as having the inherent potential of possessing a number of desirable characteristics such as high coulombic efficiency, high molecular weight for various properties, crosslinkability if desired, low cost, and versatility of composition. However, general experience with the cathodic electrocoating of such latices has revealed a number of practical deficiencies such as blistering, bubbling, loss of electrical efficiency, film roughness, and the like. The most recent proposals about cathodic electrocoating of synthetic latices made by emulsion polymerization do cite good adhesion to the electrode and obtention of desirably thin films coupled with a reasonable shutdown of current. Many of the earlier proposals for electrodeposition of synthetic latices lacked these essential characteristics, and generally such proposals were related to anodic electrodeposition of latices rather than cathodic.
In a conventional emulsion polymerization for synthesizing a latex from unsaturated monomers a few percent of residual monomer often remains, and amino fragments from initiators often are present. "Amino" is used here in the broad sense to comprehend monomers and fragments having amino, guanadino and/or amidino functionality, and, in general, nitrogenous functionality that is basically-reacting in aqueous dispersion and has molecular weight not above 300. Significant concentrations of solvents often are included in conventional cathodic electrocoating formulations in various ways. Some recent cationic-active latices have been electrodeposited unpigmented and without solvents such as alkoxyalkanols, esters of same, and hydrocarbons such as naphtha or toluene. However, it appears that there has been no recognition in the art of cathodic electrocoating with film-forming cationic latex binders, pigmented or clear, of the critical importance of precluding or eliminating nearly all of the low molecular weight (e.g., about 300 or less) amino materials and restricting the solvent content of the film-forming composition.
Advantages of the instant invention over prior proposals about the cathodic electrodeposition of latex include good pumping stability, good electrical efficiency, suppression of blistering and bubbles, increased rupture voltage, and excellent smoothness of the deposited film.